The Greek word Kyrios (Κύριος) means "lord, Lord, master". In religious usage, it is sometimes translated as God. It is used in the Septuagint translation of the Hebrew Bible and the Greek New Testament.
Kyrios appears about 740 times in the New Testament, usually referring to Jesus. The use of kyrios in the New Testament has been the subject of debate among modern scholars, and three schools of thought exist on that topic. The first is that based on the Septuagint usage, the designation is intended to assign to Jesus the Old Testament attributes of God. The reasoning here being, that at the time the Septuagint was written, when reading out loud, Jews pronounced Adonai, the Hebrew word for "Lord", when they encountered the name of God, "YHWH", which was thus translated into Greek in each instance as kyrios. And the early Christians, the majority of whom were speakers of Greek, would have been deeply familiar with the Septuagint. The second is that as the early Church expanded, Hellenistic influences resulted in the use of the term. The third is that it is a translation of the Aramaic title Mari applied to Jesus.
In everyday Aramaic, Mari was a very respectful form of polite address, well above "teacher" and similar to rabbi. In Greek this has at times been translated as kyrios. While the term Mari expressed the relationship between Jesus and his disciples during his life, the Greek kyrios came to represent his lordship over the world.
The Gospel of John seldom uses kyrios to refer to Jesus during his ministry, but does so after the Resurrection, although the vocative kyrie (meaning sir) appears frequently. The Gospel of Mark never applies the term kyrios as a direct reference to Jesus, unlike Paul who uses it 163 times. When Mark uses kyrios (e.g., in 1:3, 11:9, 12:11, etc.) it is in reference to YHWH/God. Mark does, however, use kyrios in passages where it is unclear whether it applies to God or Jesus, e.g., in Mark 5:19 or Mark 11: 3.
Kyrios is a key element of the Christology of Apostle Paul. Most scholars agree that the use of kyrios, and hence the Lordship of Jesus, predated the Pauline Epistles, but that Saint Paul expanded and elaborated on that topic. More than any other title, kyrios defined the relationship between Jesus and those who believed in him as Christ: Jesus was their Lord and Master who was to be served with all their hearts and who would one day judge their actions throughout their lives.
The kyrios title for Jesus is central to the development of New Testament Christology, for the early Christians placed it at the center of their understanding and from that center attempted to understand the other issues related to the Christian mysteries.
- κύριος. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
- The Christology of the New Testament by Oscar Cullmann 1959 ISBN 0-664-24351-7 pages 234-237 
- The Bauer lexicon, 1979 edition
- Philip Schaff. "LORD". New Schaff-Herzog Encyclopedia of Religious Knowledge, Vol. VII: Liutprand - Moralities. p. 21.
- Archibald Thomas Robertson. "Word Pictures in the New Testament - Romans".
- Mercer dictionary of the Bible by Watson E. Mills, Roger Aubrey Bullard 1998 ISBN 0-86554-373-9 pages 520-525 
- The Christology of the New Testament by Oscar Cullmann 1959 ISBN 0-664-24351-7 page 202 
- The theology of the Gospel of John by Dwight Moody Smith 1995 ISBN 0-521-35776-4 page 89
- The Gospel to the Romans: the setting and rhetoric of Mark's Gospel by Brian J. Incigneri 2003 ISBN 90-04-13108-6 pages 168-169
- II Corinthians: a commentary by Frank J. Matera 2003 ISBN 0-664-22117-3 pages 11-13
- Christology: Biblical And Historical by Mini S. Johnson, 2005 ISBN 81-8324-007-0 pages 229-235